1. Field of the Invention
The present invention relates to a dispersion-type liquid crystal electro-optical device based on a liquid crystal resin composite comprising a polymer resin having dispersed therein a liquid crystal material, and a method for forming the same. More particularly, the present invention provides a liquid crystal electro-optical device comprising substrates having set at a little spacing, yet achieving high scattering efficiency.
2. Prior Art
Many liquid crystal electro-optical devices operating on a twisted nematic (TN) or a super-twisted nematic (STN) mode using nematic liquid crystal and the like are known and put into practice. Recently, there is also known a device using ferroelectric liquid crystals. All those liquid crystal electro-optical devices basically comprise a first and a second substrate each having provided thereon an electrode and a lead, and a liquid crystal composition being incorporated between those facing two substrates. Thus, by applying an electric field between the electrodes, the state of the liquid crystal molecules having incorporated between the substrates are changed according to the anisotropy in dielectric constant. Otherwise, in the case of ferroelectric liquid crystals, the state of the liquid crystal molecules are changed according to the spontaneous polarization of the liquid crystal molecule itself. The liquid crystal devices take advantage of this electro-optical effect for displaying images.
In a liquid crystal operating in a TN or an STN mode, the liquid crystal molecules which are brought into contact with the two substrates are subjected to an orientation treatment and are arranged along the rubbing direction. The rubbing directions in the upper and the lower substrates are twisted with respect to each other so that they may make right angle or an angle from 200.degree. to 290.degree.. Thus, it can be seen that the liquid crystal molecules at the midway between the two substrates are arranged in spirals to achieve a minimum energy. In the case of an STN type device, a chiral substance is added to the liquid crystal material if necessary.
The devices described hereinbefore, however, require essentially a polarizer sheet. Furthermore, the liquid crystals must be arranged along one direction within the liquid crystal electro-optical device. Such a regular orientation of liquid crystal molecules had been achieved by rubbing an orientation film (generally an organic film) with a cotton or a velvet cloth. If not for such an orientation treatment, the liquid crystal molecules would not arrange themselves along one direction and the electro-optical effect of the liquid crystals would not be fully exhibited. Thus, a conventional device generally takes a cell-like structure comprising a pair of substrates to support the liquid crystal material therebetween, so that the liquid crystal may be injected and then imparted orientation by applying a rubbing treatment thereto to exhibit the optical effect.
In addition to the liquid crystal electro-optical devices of the type above, there is also known a dispersion-type liquid crystal capable of providing a clear and high contrast image plane, yet free of such polarizer sheets and rubbing treatment mentioned above. Referring to FIG. 2, a typical type of a prior art dispersion-type liquid crystal device is described. The two light-transmitting substrates 100 and 100' support therebetween a dispersion-type liquid crystal comprising a light-transmitting solid polymer 102 having dispersed therein granular or sponge-like liquid crystal materials 103 to give an electro-optical modulating layer. This liquid crystal device can be fabricated by dispersing encapsulated liquid crystal materials into the polymer, and then coating a film or a substrate with the resulting polymer to give a thin film. Materials such as gum arabic, poly(vinyl alcohol), and gelatin can be used for encapsulating the liquid crystal materials.
Let us consider a case in which the microcapsules are prepared by encapsulating a liquid crystal material with poly(vinyl alcohol). If the liquid crystal molecules exhibit a positive dielectric anisotropy in the polymer thin film under an electric field, the molecules are arranged in such a manner that the major axes thereof are in parallel with the electric field. Thus, if the refractive index of the liquid crystal is the same as that of the polymer, the thin film turns transparent. When the applied electric field is removed, the liquid crystal molecules take a random orientation to hinder light path, and thus the film turns opaque. Various types of information can be displayed by taking advantage of the two states, i.e., a light-transmitting and an opaque state.
Dispersion-type liquid crystals include, in addition to the encapsulated type above, those comprising liquid crystal materials being dispersed in an epoxy resin, those taking advantage of phase separation between liquid crystals and a resin by irradiating a light to a mixture of a liquid crystal and a photo-curable resin, and those comprising a three-dimensionally structured polymer having impregnated with a liquid crystal. The present invention refers to all those mentioned hereinbefore collectively as dispersion-type liquid crystals.
Because those dispersion type liquid crystal electro-optical devices can be fabricated free of polarizer sheets, an extremely high light transmittance can be achieved with the devices of this type, if compared with the conventional ones operating in a TN or an STN mode and the like. More specifically, the transmittance per polarizer sheet is about 50%, and in an device driven by an active matrix using a plurality of sheets in combination, the transmittance falls to a mere 1%. The transmittance of an STN mode device also falls to about 20%, and hence efforts are made to obtain a brighter image plane by increasing luminance of the backlighting. In contrast to these devices, a dispersion-type liquid crystal electro-optical device transmits 50% or more of the light. This owes to the fact that the device can be made completely free of polarizers.
As described in the foregoing, a dispersion-type liquid crystal functions by changing its state, i.e., a transparent state and an opaque state, and is advantageous in that it allows transmission of light at a larger amount. Thus, R & D efforts are paid mainly in realizing a transmitting-type liquid crystal electro-optical device; more particularly, in the realization of a projection type liquid crystal electro-optical device. In a projection type liquid crystal electro-optical device, the light is passed through a liquid crystal electro-optical device panel established midway in the light path of an incident light from the light source, and the light having passed through this panel is projected onto a wall via a slit having a predetermined angle. The liquid crystal molecules in the panel are in a random arrangement to give an opaque state when a low electric field below a certain threshold value is applied, i.e., when a low voltage to which the liquid crystal molecules do not respond is applied. The light is scattered upon incidence to the panel at this state, thus enlarging the light path. Then, the slit provided next to the panel cuts off most of the scattered light to give a black state to the wall. On the other hand, a light incident to liquid crystal molecules having arranged in parallel in correspondence to the applied electric field passes straight through the molecules without being scattered to yield a light state on the wall at a high brightness.
It can be seen from the foregoing description that a dispersion-type liquid crystal switches its state in accordance with the light incident to the light-control layer (electro-optical modulating layer) through a light-transmitting substrate. The light inside the light-control layer (electro-optical modulating layer) changes its light path for a plurality of times each time it comes to a boundary between the resin and the liquid crystal droplets having dispersed therein, and reaches the substrate on the other side in a greatly scattered state. Thus, to obtain a high scattering efficiency, the resin and the liquid crystal droplets are preferably arranged as such that they contact with each other as many times as possible along the thickness direction of the light control layer (electro-optical modulating layer), because the light is scattered each time it comes to the boundary between the droplet and the resin. Of course, the scattering efficiency also increases with increasing thickness of the light-control layer (electro-optical modulating layer). However, a thicker control layer (electro-optical modulating layer) also increases the distance between the substrates, i.e., the distance between the electrodes. As a result of this extended distance between the electrodes, a larger driving voltage for switching the light control layer (electro-optical modulating layer) becomes necessary. Thus, despite an improvement in scattering efficiency is achieved, the control layer (electro-optical modulating layer) then cannot be driven with an integrated circuit (IC), i.e., a thin film transistor (TFT).